Nitriding stopoff



NITRIDING sroPoFF Joseph H. Dew, Flint, Mich., assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware No Drawing. Application April 6, 1953, Serial No. 347,155

1 Claim. (Cl. 148-164?) This invention relates to the surface hardening of metals, and more particularly to improvements in selective nitriding operations.

In typical nitrogenization processes the surface of a metallic article is exposed at an elevated temperature, generally within a temperature range of from about 900 F. to 1150 F., to a nitrogenous atmosphere, the metal absorbing nitrogen to form at its surface a diifusion zone of metallic nitrides. The surface of a metal so treated can absorb several percent of nitrogen in the form of nitrides, either of iron or of certain other elements which may be present to form a nitrided diffusion zone which decreases in nitrogen content until it finally merges with the unchanged core. As a result of the nitriding operation, the hardness of the metallic surface is considerably increased.

The nitrogen atmosphere preferably is supplied in the nascent form, by utilizing a nitrogen-containing gas such as ammonia which is readily dissociated by hot metal surfaces to liberate nascent monatomic nitrogen.

Nitriding operations are particularly useful in surface hardening various types of iron and steel. Steels subjected to commercial nitriding usually are supplied in a wrought, quench-hardened, and tempered I condition, ready for machinin g'and/or grinding prior to nitriding. It is desirable to remove any decarburization (usually metal is removed) which has occurred prior to nitriding to insure sub-surface hardness necessary to support the hard nitride case. Relief of internal stresses, induced in rough machining operations, prior to nitriding is desirable This stress relief is a precautionary measure tominimize distorsion during the nitriding process. Frequently used stabilizing treatments to reduce the internal stresses include soaking the metal for a period of time at an elevated temperature of at least 50 F. above the maximum temperature to be used during subsequent nitriding. The stabilizing temperature, however, generally should not exceed the temperature at which the metal is tempered, in order to preclude any modification of the core properties.

For superior results, an article to be nitrided should have a smooth finish and a clean surface. In many instances the nitrogen absorptivity of the surface is improved by subjecting the article to a degreasing, sand blasting, and bonderizing treatment.

Nitriding operations as described thus far have heretofore been used to provide a nitrided zone over the entire surface of a metallic article. However, in many applications the formation of a nitrided case over the entire surface of the article is highly undesirable. For example, in the formation of machine gears, while it is desirable to nitride-harden the involute wear surfaces of the gear teeth, it is equally important that the re mainder of the gear be unhardened. However, up to the present time, certain difficulties have been encountered in nitriding only selected portions of a metal article such as the aforementioned gear teeth wear sur- 2,788,302 Patented Apr. 9, 1957 faces. Prior methods of selectively nitriding portions of a metallic surface generally have involved applying to the article an initial electroplated or dipped coatingof copper, bronze or similar metal which is resistant to penetration by nitrogen. Portions of this protective coat ing are subsequently removed by machining or the like to expose the surfaces which are to be nitrided. In many applications however, particularly in high production operations, such an initial electroplated or dipped coat-' ing does not provide the desired protection. Because of rough handling, and/or poor adherence, such a pro-- tective coating is often scratched, torn or, in some cases, peeled away, thus exposing to the nitrogen atmosphere a surface which should not be nitrided. In the past, some attempts have been made for touching up or patch-- ing such defects in an electroplated or dipped protec tive coating. However, up to the present time, the materials employed have proved unsatisfactory in many nitriding operations and particularly in selective nitriding operations since such compounds tend to run or migrate when exposed to the nitriding temperatures, hence rendering accurate control of the areas to be nitrided practically impossible.

Accordingly, one object of this invention is to provide an improved nitriding stopotf composition which permits accurate control of selective nitriding operations. A further object is the provision of a stopoff composition which can easily be applied and which will not migrate at nitriding temperatures. A further object of the invention is the provision of an improved composition which can be utilized as a protective coating to preclude absorption of nitrogen by a metal to which it is applied. a

The present invention contemplates using as a protective coating in metal hardening operations a composi-' tion comprising a mixture of a powdered metal and a carrier medium. I have discovered that excellent results are obtained in selective nitriding operations by employing as a stopofi composition a mixture comprising powdered metal and finely divided refractory material including, in many instances, a small amount of binder such as sodium silicate. Illustrative of the type of refractory materials contemplated are such materials as kaolin, alumina, silica sand, olivine, magnesite, various clay materials of the aluminum or magnesium silicate type, as well as mixtures thereof with consistency reducers and/or heat-resistant binder materials. Superior results are obtained in many instances by employing a stopoif composition comprising a mixture of a major proportion of finely divided tin and kaolinite, and a minor proportion of sodium silicate and water.

The proportions of the various ingredients may be varied somewhat according to the particular application intended. The refractory material, which serves to provide sufiicient coating porosity at nitriding temperatures to hold the tin in solution and to facilitate even tin coverage of the surface to be protected, generally should comprise approximately 30% to by volume of the composition. A preferred composition comprises a mixture of about 50% by volume of refractory material and 50% by volume of tin powder although in particular applications the tin content may be as low as about 30% by volume.

The proportions of refractory ingredients in the mob position also may be varied somewhat. At present, however, I prefer to employ a refractory mixture comprising about 50% to 70% by weight of comminuted kaolin or similar clay type material, up to about 5% by weight of sodium silicate, and the balance water.

The following are illustrative examples of stopott compositions embodying the present invention:

3 Example 1 A stopofi coating composition. is prepared by mixing together equal volumes of finely divided tin and a refractory mixture having the following composition expressed in percent by weight:

Percent Kaolinite 59.3 Sodium silicate 2.3 Water 38.4

Example 2 A stopott coating composition is prepared by mixing together 30% by volume of powdered tin and 70% by volume of a refractory mixture having the following composition expressed in percent by weight:

Percent Potters clay 50.0 Sodium silicate 1.0 Water 49.0

Example 3 A stopolf coating composition is prepared by mixing together 60% by volume of finely divided tin and 40% by volume of a refractory mixture having the following composition expressed in percent by weight:

Percent Kaolinite 60.0 Sodium silicate 2.0 Water 38.0

While the above composition ranges and examples indicate generally the preferred proportions, at times it may be desirable to increase the metallic content up to about 75% by volume. It will be understood of course that in particular applications the water can be replaced with some other liquid medium such as tuipentine, butyl acetate, zylene, toluene or similar organic solvent or diluent when a more rapid evaporation is desirable.

A stopoif composition as hereinbefore described may be used to repair imperfections in a previously applied electroplated or dipped coating of copper, bronze or the like, or as an initial protective layer in lieu of any metallic coating. A composition embodying the present invention is particularly advantageous when it is necessary to accurately restrict the area to be nitrided. Such a composition does not run or migrate even under the high temperatures encountered in commercial nitriding operations. Hence, the areas to be nitrided can be defined with great accuracy.

A particular example of a nitriding operation employing the composition of the present invention is the following: A steel article is first bronze plated using conventional plating procedures. then machined away where it is desired to provide a nitrided case. In commercial operations this invariably results in damaging the bronze coating where nitriding is to be prevented. A composition comprising one part by volume of tin powder and two parts by volume of a mixture comprising 59.3% by weight of kaolinite, 2.3% by weight of sodium silicate, and 38.4% by weight of water is brushed on as a stopoff coating for the areas from. which the bronze plating has inadvertently been removed. The thus coated article is then exposed to a nitrogen atmosphere at a temperature of approximately 950 F. for about 10 hours. This treatment is followed by a 30 hour diffusing cycle at a temperature of about 1050" F.

From the foregoing description it will now be understood that the present invention provides a composition permitting the accurate control of selective nitriding operations'. A composition embodying the invention can be easily applied to a surface which is to be prevented from absorbing nitrogen.

It is understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

The method of hardening by nitriding selected portions of a metal article which comprises coating the remaining portions of the metal article with a stopoif composition consisting of a mixture of about 30% to 75% by volume of finely divided tin and about 25% to by volume of refractory material, said refractory material consisting of a mixture of 50% to by weight of kaolin, a small but efiective amount of sodium silicate not over 5% by weight and the balance water, and thereafter subjecting said coated metal article to a nitriding atmosphere under elevated temperature conditions.

References (Iitcd in the file of this patent UNETED STATES PATENTS 1,980,670 Eckman et a1. NOV. 13, 1934 2,196,232 Sweeney Apr. 9, 1940 2,367,978 Troy Jan. 23, 1945 OTHER REFERENCES Metals Handbook, American Society for Metals, 1948 edition, pp. 700-701.

The bronze coating" is 

